With nearby (and smaller) Alphonsus
and Arzachel, this is huge "circular formation" is one of the most
interesting and impressive vistas on the Moon. Ptolemaeus is 93-miles in diameter,
and features a flat floor that includes ghost craters and many small craterlets.
The largest of these floor craters is Ammonius, which is a young-looking 5-mile-across feature. Beside Ammonius is the barely perceptible Ptolemaeus B, which
was drowned in the lava flow that created Ptolemaeus' smooth floor. The north
wall of Ptolemaeus is graced by the complex and well-defined Herschel (named
after Sir William Herschel, natch).